/**
 * 截取字符串的函数还有 substr substring
 * @param {*} start 
 * @param {*} end 
 * @returns 
 */
String.prototyep.mySplice = function(start,end){
    if(start<0){
        start=start+this.length
    }
    if(end<0){
        end=end+this.length
    }
    if(!end){
        end=this.length
    }
    let str=''
    for(let i=start;i<end;i++){
        str=str + this[i]
    }
    return str
}